<i>Background:</i> We undertook this study to characterize those patients who required reoperations for Chiari malformation and to determine whether modifications in surgical technique at the initial procedure might have obviated the need for repeat surgery. <i>Methods:</i> We reviewed the hospital records, imaging studies, operative reports, and follow-up data of those patients who were undergoing a second operation as part of a consecutive series of Chiari decompressions carried out by one neurosurgeon over a 14-year period. We wished to determine whether type of Chiari malformation, age at initial surgery, the presence of associated bony anomalies, or initial operative technique contributed to the need for reoperation. <i>Results:</i> Twenty-two (17%) of the surgical procedures represented reoperations (16 of 100 Chiari I and 6 of 33 Chiari II decompressions). Of the patients in the series who were operated on at age 5 years or younger, 7 (16%) were reoperations (5 of 33 Chiari I and 2 of 10 Chiari II decompressions); 15 patients (17%) of those older than 5 years at surgery were reoperations (11 of 67 Chiari I and 4 of 23 Chiari II decompressions). 8 of the 16 patients who underwent reoperations for Chiari I malformation and 4 of the 6 patients who underwent reoperations for Chiari II malformation had their initial decompression surgery at age 5 years or younger. Reoperation was required because of failure to treat adequately an associated syrinx (n = 11), because of persisting neurological symptoms or headache (n = 9), or because of a possible faulty fourth ventricular stent placement (n = 2). 5 of 9 patients in the operative series with Chiari malformations associated with craniosynostosis represented reoperations (55%), and all 3 of the patients whose syrinx persisted after reoperation had craniosynostosis syndromes. <i>Conclusions:</i> Approximately 17% of the patients with Chiari I and Chiari II malformations in this operative series were undergoing a second operation. Factors in this series which were associated with reoperation included a young age at initial surgery, the presence of complex bony anatomy at the foramen magnum, syndromic craniosynostosis, and failure of the surgeon at the initial operation either to assess patency of the foramen of Magendie or correctly place a fourth ventricular stent.